The fovea is a highly specialized retinal region that allows a normal visual system to attain better than 20/20 visual acuity. When an individual looks at an object, that object is imaged onto the fovea of the eye. The fovea is surrounded by a uniquely arranged radial pattern of birefringent Henle fibers, fibers that change the state of polarization of transmitted light. The birefringence of these Henle fibers has been exploited to detect true foveal fixation of an eye, by means of retinal birefringence scanning (RBS). In RBS, a spot of polarized near-infrared light is scanned on the retina, most simply in a circle with a fixation point in the center, and the changes in the polarization of light returning from the eye are analyzed to detect the projection into space of the Henle fibers surrounding the fovea.
Due to the radially symmetric arrangement of the birefringent Henle fibers surrounding the fovea, a characteristic frequency (or more than one frequency, depending on the opto-mechanical design) appears in the obtained periodic signal when the scan is exactly centered on the fovea, indicating central fixation. Thus, by analyzing the generated frequencies in the obtained RBS signal, for example by means of the Fast Fourier Transform (FFT), the goodness of eye fixation can be assessed.
By detecting the radial symmetry of foveal architecture, RBS directly assesses true foveal fixation of the eye and does not require eye-gaze calibration such as other methods of eye fixation detection. This advantage makes it possible to investigate less cooperative subjects, including young children at risk for developing amblyopia (“lazy eye”), which is the leading medical cause of decreased vision in childhood. Binocular RBS has the potential to screen young children for strabismus (misalignment of the eyes), the most common cause of amblyopia. Currently available vision screening devices can detect strabismus only indirectly via asymmetry of the positions of the corneal light reflexes.
RBS has demonstrated reliable and non-invasive detection of foveal fixation, as well as detection of strabismus. However, as with all polarization-sensitive technology used for retinal scanning and other intraocular assessment, RBS is adversely affected by corneal birefringence, which contributes most to the overall ocular birefringence and varies widely in both its amount (corneal retardance) and orientation (corneal azimuth) from one eye to the next, and across an individual pupil, thus creating variability in the RBS signal levels from one eye to the next and occasionally masking the desired signal from retinal birefringence.
Opto-mechanical designs that use wave plates (“wave-plate-enhanced RBS”) and other optical components that manipulate the polarization of light can be used to enhance foveal fixation detection while minimizing the deleterious effects of corneal birefringence in retinal birefringence scanning. In such a system, a double-pass half wave plate (HWP) spinning at a specific fractional frequency of the scanning frequency (f), more precisely at an odd multiple of 1/16th as fast as the scanning frequency [( 9/16)f] is used, generating so-called “multiple-of-half” frequency components from birefringent patterns on the retina. For example, in one arrangement incorporating a double-pass fixed conventional wave plate in addition, 2.5f and 6.5f frequencies are generated with central fixation, and 3.5f and 5.5f frequencies are generated with off-center fixation. In addition, a high-amplitude 4.5f “spinning artifact” frequency is generated from interaction between the spinning half wave plate, the fixed conventional wave plate, and the corneal birefringence. A schematic design of this arrangement is illustrated in FIG. 1.
In the past, the azimuth and retardance of the double-pass fixed conventional wave plate were optimized for a large set of eyes with corneal birefringences representative across the population, considering only one of the predominant frequency components indicating central fixation (2.5f in that case), by calculating the minimal normalized standard deviation (standard deviated divided by the mean) of RBS signal strengths (FFT power) at that frequency (2.5f), and the wave plate with the minimal normalized standard deviation of RBS signal strengths at 2.5f was chosen to identify the best retardance/azimuth combination for the fixed wave plate to be added to the RBS system.
However, optimizing the fixed wave plate considering only one of the predominant frequency components results in a spinning artifact (4.5f signal in this particular opto-mechanical configuration) that is by no means uniform over the population range of corneal birefringence (see FIG. 2). While such optimization may generate results described as independent of fixation, independent of the state of eye fixation, independent of the fixation condition of the eye, the results are not independent of corneal birefringence! All eyes yield a very high to extremely high signal level for the spinning artifact frequency, but this level can vary significantly with the azimuth and retardance of the corneal birefringence of the eye that is measured. In other words, the spinning artifact is dependent on the corneal birefringence of an eye, that is, it is a function of corneal birefringence. A varying spinning artifact level can thus not properly be used for normalization of the RBS signal strengths. Also, if a varying signal level of the spinning artifact frequency is used to assess the focus of the eye during RBS testing, the signal-to-noise ratio of the focus signal would vary with the given eye's corneal birefringence, distorting the signal quality from one eye to the next. Thus to be used for these important purposes (normalization and “independent” focus assessment), the signal level of the spinning artifact frequency should be relatively independent of both the fixation condition and the corneal birefringence of the eye.
It would therefore be advantageous to provide a retinal birefringence scanner that is capable of providing a spinning artifact frequency signal level that is relatively independent of both the fixation condition and the corneal birefringence of an eye, in order to determine the goodness of eye fixation and eye focus.